1. Field of the Invention
The present invention relates to a mobile communication system including a first communication system and a second communication system, a network device provided in a mobile communication system, and a mobile communication method.
2. Description of the Related Art
An Inter-RAT handover is known as a procedure for a mobile communication terminal (UE) to switch from the connection with a first communication system (first Radio Access Technology (RAT)) to that with a second communication system (second RAT) in a mobile communication system.
With respect to the Inter-RAT handover, a technology (Forwarding technology) is known in which data (packet) having been already received by the first communication system is forwarded from the first communication system to the second communication system when switching (Path Switch) from the first communication system to the second communication system is determined (for example, Japanese Patent Application Publication No. 2008-041329 and Japanese Patent Application Publication No. 2008-236269).
Moreover, with respect to the Inter-RAT handover, a technology (Reordering technology) is known in which data forwarded from the first communication system (hereinafter, forwarding data) and data which the second communication system directly receives from a core network (hereinafter, direct data) are reordered.
In the Inter-RAT handover, an end marker indicating the last forwarding data forwarded from the first communication system is used. The second communication system is configured to start transmission of the direct data when the end marker is forwarded from the first communication system.
A path through which the forwarding data is forwarded between the first communication system and the second communication system is not a control plane (C-PLANE) but a user plane (U-PLANE).
The first communication system (transmission side) can acquire an IP address and a PORT number of a device provided in the second communication system (reception side). Thus, the first communication system can detect whether the forwarding path of the forwarding data is normal or not by transmitting a packet requesting a reply (for example, ECHO packet) to the second communication system.
However, the second communication system (reception side) cannot acquire an IP address and a PORT number of a device provided in the first communication system. Thus, the second communication system cannot transmit the packet requesting a reply (for example, ECHO packet) to the first communication system, and cannot detect whether the forwarding path of the forwarding data is normal or not.